The administration of non-invasive CPAP (Continuous Positive Airway Pressure) is an effective way of treating patients who suffer from OSA (Obstructive Sleep Apnea) and upper airway resistance syndrome. CPAP treatment effectively acts as a pneumatic splint of the patient's upper airway. Common to all forms of non-invasive CPAP apparatus is a nose, mouth or face mask which is fitted to a patient and connected to a flow generator via a flexible air delivery tube/conduit. The flow generator includes an electric motor driving a turbine to supply air or breathable gas for the administration of CPAP treatment to the patient during sleep. The positive air pressures supplied at the entrance to the patient's airway typically is in the range 2-20 cm H.sub.2 O.
In bilevel CPAP, the pressure of the delivered air or breathable gas is ideally switched between two levels coinciding (in synchronism) with patient breathing. The pressure required to maintain adequate airway patency is typically substantially higher in inspiration than in expiration. Further, the pressure level required during inspiration is approximately equal to the single fixed pressure level used in CPAP therapy. This observation permits the administration of a lower pressure (referred to as EPAP) in expiration, and a higher pressure (referred to as IPAP) during inspiration. Therefore, the mean pressure delivered to the patient is reduced compared with CPAP therapy, leading to increased comfort and potential compliance. In some instances bilevel CPAP may also be used to provide respiratory assistance or ventilation. Much of the practical difficulty in designing CPAP apparatus is the accurate detection of the transition between inspiration to expiration so that synchronism with respiration is maintained.
When bilevel CPAP treatment is employed using only a nose mask, for some patients the higher IPAP pressure can introduce a mouth leak by which air entering the nose escapes via the mouth. The presence of a mouth leak during IPAP makes it difficult for the CPAP apparatus to accurately detect when the patient exhales. The IPAP pressure therefore may erroneously be maintained during expiration, thereby increasing the work of breathing, possibly leading to an arousal from sleep.
One method of circumventing the problem of mouth-leak is to use a full face mask or a combined nose/mouth mask; however this may lead to discomfort for some patients and effectively sealing the mask is difficult.
An alternate manner of minimising the effect of mouth-leak is to limit the maximum time for which the CPAP apparatus can remain in the IPAP state. With an appropriate limit on the duration of the IPAP time, the machine eventually `times-out` and reverts to EPAP treatment pressure if it is unable to detect, as a result of a leak, that the patient has exhaled. When the patient next inhales, the CPAP apparatus detects this occurrence and reverts to the IPAP treatment pressure.
In all known bilevel CPAP apparatus the IPAP time-out is of a fixed duration, typically three seconds, which is longer than the usual maximum inspiratory time. From clinical trials conducted by the present inventors, it has become apparent that problems still arise, as it is possible for a patient to take a number of breaths before the time-out occurs. The patient must therefore still breath against the IPAP pressure, so the work of breathing is increased. The benefits of the delivered therapy are therefore diminished and, in some cases, the device may act to the patient's detriment.
The present invention is directed to overcoming or at least ameliorating one or more of the above-mentioned disadvantages.